Kobe, Japan (CNN) -- There is a reason more than half the world's population lives in cities, with the number expected to grow. Cities have a lot to offer. Residents can walk to nearby shops and enjoy cultural attractions not available to those in more rural areas. Also, living in a city may make your commute to work much shorter.

Unfortunately, according to health officials from the World Health Organization, that convenience may come with a price -- higher levels of stress and a measurable impact on your brain.

The problem seems to be "attention," or more specifically, the lack of it. With so many different distractions -- from a flashing neon sign, to the cell phone conversation of a nearby passenger on a bus, a city dweller starts to practice something known as "controlled perception." That toggling back and forth between competing stimuli can be mentally exhausting.

In fact, according to a recent study from the University of Michigan in Ann Arbor, just living in an urban environment makes it more difficult for an individual to hold things in memory.

In the same study, researchers split undergraduate students into two groups. One spent the day in a suburban neighborhood, the other group in a busy city. Overall, those in the city scored lower on attention tests and had a worse mood comparatively.

Some of this may be no surprise, especially if you live in a city yourself. The good news, however, is a solution may be relatively easy. Recent studies have shown just getting glimpses of green areas can improve brain performance.

While it would be great to get completely "unplugged" for long periods of time -- spending a few minutes a day in a park gives you time to reduce your cognitive efforts and relieve mental exhaustion.

So, a key for urban city planners is to maintain and enhance natural green settings as much as possible, and for individuals to take brief respites to enjoy them.